White, biaxially oriented polyester films are known in the prior art.
DE 26 47 713 C2 (equivalent to U.S. Pat. No. 7,090,915 B2) describes a biaxially oriented opaque photographic layer substrate which is comprised of a linear polyester and comprises an amount of from 5 to 50% by weight of barium sulfate, as sole particulate pigment. The average grain size of the barium sulfate particles is from 0.5 to 10 μm. A feature of the biaxially oriented opaque photographic layer substrate is particularly high whiteness and particularly high opacity, without any discoloration of the layer substrate as a consequence of additives present therein, during production. It also has low density, good specular gloss, and low overall light permeability. However, the film retains shortcomings in its production (non-ideal presentation of the roll) and in optical properties.
EP-A-1 125 966 describes a white, opaque film whose thickness is in the range from 10 to 500 μm, which comprises a crystallizable thermoplastic as main constituent, and which is characterized in that the film comprises at least barium sulfate as white pigment, at least one UV stabilizer as light stabilizer, at least one flame retardant, and at least one optical brightener. The proportion of barium sulfate in the film is in the range from 0.2 to 40% by weight, and that of the UV stabilizer as light stabilizer is in the range from 0.01 to 5% by weight, and that of the flame retardant is in the range from 0.5 to 30.0% by weight, and that of the optical brightener is in the range from 10 to 50 000 ppm, based on the weight of the thermoplastic. The varied combination of properties makes the film suitable for various applications, for example for interior cladding, for construction of exhibition stands, and for exhibition requisites, for displays and for placards, for labels, for protective glazing of machinery and of vehicles, in the fitting-out of shops and of stores, as a promotional item or lamination medium, and in food-and-drink applications. However, the film retains shortcomings in its production (non-ideal presentation of the roll) and in optical properties.
EP-A-1 125 937 relates to a white, opaque film with low transparency comprised of a crystallizable thermoplastic, the thickness of the film being in the range from 10 to 500 μm. The film comprises at least barium sulfate as white pigment, and also an optical brightener, and features good orientability, low transparency, and also very good optical and mechanical properties. The barium sulfate and/or the optical brightener are either incorporated into the thermoplastic directly at the premises of the polymer producer or are fed in the form of a masterbatch during film production. The combination of properties of the film makes it suitable for applications comparable with those described above for EP-A-1 125 966 (equivalent to U.S. Pat. No. 7,090,915 B2). However, the film retains shortcomings in its production (non-ideal presentation of the roll) and in optical properties.